This invention relates to a semiconductor device prevented from changes of internal stress in a silicon thin film and generation of crystal defects caused by the changes of internal stress, and processes for producing the same, as well as to processes for producing a silicon thin film and a chemical vapor deposition apparatus suitable for forming such a silicon thin film.
In the production of semiconductor devices, a silicon thin film is used as electrodes and/or a wiring material. Since the silicon thin film is a semiconductor material, it is necessary to reduce electric resistance when used as a wiring material. In general, it is doped with an element of group III or V (e.g. B, P, As, etc.) by diffusion. In the doping with such an impurity, there has been employed thermal diffusion from film surface or ion implantation.
Recently, since the structure of semiconductor devices is complicated, a level difference of surfaces on which the thin film is to be deposited is made as small as possible in order to improve evenness of deposition of the thin film. Thus, there is a tendency to reduce the film thickness of various thin films including a silicon thin film. When the film thickness is reduced, there arise problems such as contamination of an underlying film with a dopant, concentration and uneven deposition of a dopant near the interface of underlying film, and the like, when the thermal diffusion from film surface or the ion implantation is employed. In order to solve such problems, an in-situ doping technique wherein an impurity is doped simultaneously at the time of deposition of a silicon thin film is proposed and used for producing products.
As processes for depositing a silicon thin film, there are known a process which comprises depositing silicon in an amorphous state, followed by polycrystallization by heat treatment, and a process for depositing in a polycrystalline state from the beginning. Generally speaking, since there is a tendency to enlarge crystal grain size in the case of deposition in an amorphous state, followed by polycrystallization by heat treatment, it is preferable to form a polycrystalline silicon film by this process in order to attain low electric resistance of the thin film. Therefore, there is widely used a process for forming a polycrystalline silicon film comprising depositing amorphous silicon doped with an impurity simultaneously, followed by polycrystallization by heat treatment. Such a technique is disclosed, for example, in Japanese Patent Unexamined Publication No. (JP-A) 62-54423 and 4-137724.
But, according to such a technique, there are following problems. When an amorphous (including a fine crystalline state) silicon thin film is crystallized by heat treatment, it is generally known that crystal nucleuses are grown from the interface between the silicon thin film and the underlying film. Therefore, the state of crystal growth is often changed (by, for example, generating density and generating temperature of crystal nucleuses, crystal grain size, or growing crystal plane indices) depending on an impurity concentration or its distribution in the amorphous silicon film near the interface of underlying film.
Further, at the time of crystallization reaction, since the volume of thin film is changed, the internal stress state in the film is also changed greatly. Further, the direction of stress (i.e. tensile strength or compression stress) generated at the time of crystallization is greatly changed by growing crystal state. As a result, there arise various problems in that generated internal stress in the silicon thin film becomes greater, or in a wafer on which the thin film is deposited, the internal stress in the thin film and growing crystal planes are differentiated, the degree of concentration of stress generated near end portions of the thin film and the crystal state are also differentiated, crystal defects such as dislocation are generated in a silicon single crystal substrate, electrical properties of a semiconductor device are differentiated in a wafer including a silicon single crystal, etc.